1. Field of the Invention
The invention concerns an apparatus for the exposure of photosensitive plates on both sides. More particularly, the invention concerns the preparation of printed circuits for electronic devices or the preparation of light sensitive printing plates wherein two exposure devices are arranged adjacent each other. Each exposure device is provided under the exposure frame with an illumination system and with a glass plate filling the format being arranged in the exposure frame. A positive or negative copy may be placed, on the glass with the photosensitive plate above it for the exposure process.
2. Background of the Art
Photosensitive plates, in particular for the preparation of printed circuits for electronic circuits, comprise a support layer, preferably of a composite material made of an insulating substance and metal foil, mainly copper, upon which a layer of a photosensitive synthetic plastic layer is placed by means of a layer of an adhesive. Hardening takes place in the locations of the plastic layer exposed to light by the formation of polymers, while the unexposed parts are dissolved out after the exposure process, to the support material. The exposure of the synthetic plastic layer is brought about by placing a negative or a positive copy of the same size directly onto the photosensitive plate. Predetermined exposure times are required by means of UV light sources located below the photosensitive plate. The term "exposure" signifies herein that the locations of the resist are polymerized due to the action of the UV light on the synthetic plastic layer exposed through the master. In the process the monomer components are cross-linked and the resulting polymer is insoluble in the subsequent developing process. In order to obtain an exact reproduction of the master, it is necessary to use a light beam as parallel as possible. In addition, the master must be aligned accurately on the base material for proper reproduction.
In the preparation of printed circuits the objective is to carry the resolution capability as far as possible so that reductions in size of the electronic circuits become possible. In the copying process to obtain high resolutions all interferences for example by dust particles must be eliminated. For this and other reasons (to be mentioned later) "contactless" copying is desirable. In the process, a distance of approximately 1/10 to 2/10 mm is to be maintained between the master and the photosensitive material so that a flow of air may be passed through the gap by means of a suitable blower. In this fashion the settling of dust particles is prevented. "Contactless" copying further tends to preserve the master so that a practically endless number of copies may be produced from a single negative or positive master. However, to obtain "contactless" copying it is necessary to provide an incident light beam of the source of light that is as perpendicular as possible.
In the case of plates with large surface areas to be exposed, this requires that the source of light be removed from the plate to be exposed as far as possible. In this manner, a more uniform exposure of all areas of the surface is also obtained.